"...We know how much money has been flowing into those bonds funds over the last three or four years. All of a sudden they might get a little nervous and say where am i going to go? Where can i get some yield and also some protection against inflation and growth? and that's when I think we're going to see people fleeing the bond market moving into stocks."

And Siegel isn't too concerned about threats from Europe, oil prices or the Middle East:

"I hear all these problems. But history says the greatest bull markets climb the wall of worry. There's always these things you can worry about to keep people out Michael when no one has any worries when there's no clouds on the horizon that's when I want to get out of the market."

Siegel also said that oil prices are a wild card but the U.S. is half as sensitive to oil now as it was back in the 70s.